The present invention relates to a developing system for use in an electrostatic recording apparatus such as an electrophotographic copying apparatus.
Developing systems of this kind are already known in which a toner supply hopper is provided on one side or the other side of the width of an area to be developed. The hopper supplies toners in response to a toner replenishment signal when the density of toners or the density of an image pattern is low. As the toners thus supplied are conveyed along a gutter-like toner conveyance guide, i.e. a receptacle in the direction of the width of the area to be developed by means of a spiral screw, the toners are replenished through toner dropping holes provided substantially under the conveyance guide. In the meantime, toners running along the conveyance guide are formed in fine particles of the order of 1-20 .mu. and are insufficient in fluidity. Therefore an attempt has been made to improve the characteristics of both conveyance and dropping such toners through the toner dropping holes in the manner that a part of developers are moved or brought into the conveyance guide by means of a developer agitator provided inside the developing system and are then mixed up with the toners therein. In this case in particular, the developers are conveyed in one direction with respect to the developing system. Therefore, the developers tend not to be uniformly distributed in width direction.
As a result of the above difficulties, the inventors studied a system making use of a primary spiral screw and a secondary spiral screw mounted below the primary spiral screw, the secondary spiral screw conveying developers in the opposite direction to the conveyance direction of the primary spiral screw, thus moving the developers in both directions so that the developers may be evenly distributed. It was found that the quantity of developers inside the conveyance guide, i.e. the quantity of developers added thereinto, is limited in order to drop the replenished toners evenly in one direction, i.e. in the development width direction. Such limitation is apt to occur particularly when a developing system is small in size. In other words, if the quantity of the developers inside the conveyance guide is increased to increase the quantity of the developers conveyed to one side of the development width, i.e. in the direction of conveyance, the toner density on said one side becomes substantially higher. On the contrary, if the quantity of developers inside the conveyance guide is reduced to reduce the quantity of such developers conveyed to one side of the width, the toner density on the other side becomes substantially higher. As a result, the density distribution of the toners inside the developing system becomes uneven so that a corresponding unevenness of the density on an image pattern is apt to occur.
Possible solutions to the above-mentioned problem are to mix up replenished toners with a large quantity of developers and then drop the mixture thereof from a conveyance guide; or to improve the agitation capacity in the development width direction. In the former case, the time-lag of toner replenishment becomes longer, while in the latter case, it is unsuitable for reducing the cost of a copying apparatus and miniaturizing the size thereof.
It is an object of the invention to provide a developing system in which the above-mentioned disadvantages are eliminated; the even density of toners is obtained in a development width direction; and an excellent image pattern is reproduced even if such copying apparatus is low in cost and is small in size.